The present invention is an apparatus for controlling the ventilation of material to be dried in a tunnel drying plant, more specially for green architectural ceramic structures, such as bricks or tiles, which are moved through the drying plant in a direction opposite to an air current.
In a tunnel drying plant of this design with a tunnel breadth and tunnel height of some meters, carriages with a size matching the size of the tunnel are moved along it from end to end with the material to be dried on them in layers or stories. The air needed for taking the moisture from the material to be dried is moved through the tunnel drying plant or drying kiln in a direction opposite to the direction of motion of the carriages. Drying plants are known in the prior art in which forced draft heating units are placed along the length of the tunnel, or in which the air is completely or partly removed from the drying tunnel and then let into it again.
The speed of the drying air through a counter-current tunnel drier of the sort noted is about 0.5 to 2.0 m/sec, the air, because of its being heated, moving more specially along the tunnel in the top part of its cross-ection so that goods to be dried in the lower part of the cross-section and in the middle parts of the tunnel will only be dried at a very slow rate.
For stepping up the production throughput of a tunnel drying plant, a suggestion has been made in the prior art to have circulating fans for the dry air along the drying tunnel for circulation of the air across the tunnel cross-section at a speed which is many times higher than the speed at which the air would have without such fans, that is to say simply due to its being forced into and let out from the tunnel, the purpose of such fans being that of evening out the temperature and the moisture level over the tunnel cross-section and acting on the goods to be dried with dry air.
Because, on continuous ventilation of the goods to be dried, water is more quickly evaporated from the outer face of the goods than such water may be replaced by water moving through the capillary spaces in the goods being dried, the goods will be dried out quickly at the outer face and such outer face or skin of the goods will undergo shrinkage at a higher rate than inside the same. For this reason the goods to be dried may be damaged by cracking, spalling and other effects.
For taking care of this shortcoming a tunnel drying plant has been designed in which the circulation fans or blowers are designed so that they may be moved between the trains or lines of goods carriages so that, by means of them, the goods are dried in line with a given program. This puts an end to the shortcoming we have noted with continuous ventilation and the carriages are furthermore ventilated between the circulation fans.
Furthermore, a tunnel drying plant with a low tunnel height has been put forward which only a low layer of goods to be dried, roughly 0.2 to 0.5 meter, is dried in broad counter-current tunnels, the circulation fans being fixed in position, or being able to be moved, over the goods.
With known low level or low-height tunnel drying plants a very much higher throughput speed is produced than is the case with a high-volume tunnel drying plant, the air speed in tunnel drying plant amounting to up to 12.0 m/sec.
Because of this high air speed, it is hard to control the moisture gradient in the tunnel drying plant so that with a such tunnel drying plant only more or less sturdy goods may be dried.
While the shortcoming of a tunnel drying plant with circulation fans is to be seen in the high power need for ventilation of the goods, a tunnel drying plant of low height without forced circulation likewise has a shortcoming inasmuch as there is a high air speed or velocity, which in itself makes the control of the drying process very much harder than in other plant.